The present invention relates to a system which transmits torque from a drive shaft to a tubular member that has the capability of preventing excessive torque being applied, particularly when the driven mechanism becomes frozen due to mechanical failure, overload, or the like. The present application is particularly suitable for incorporation in a drive system for a conveyor roller with an input or inputs being received from a motor driven motor or drive shaft. It will be understood that in the typical conveyor system, one or more of the rollers is power driven while the remainder are free rolling. Whenever the articles being carried by the conveyor become jammed or overloaded, time must be allowed for the operator to relieve the jammed condition without forcing a continuing drive to the conveyor in such manner as to injure the articles or destroy the drive mechanism itself. The prior art shows a number of torque limiting clutches and the like. These generally require rather complex and costly mechanisms and they are not easily adjustable to selectively vary the drive torque in accordance with the load to which the conveyor may be subjected.
Another approach that has been made to provide a torque limiting coupling is exemplified by U.S. Pat. No. 3,942,338, issued of Furlette, et al on Mar. 9, 1976 for "Torque Limiting Coupling". The combination taught by that patent is that of a drive shaft and a hollow, tubular driven member in which the torque communicating element between the tube comprises a hollow, elongated sleeve. That system is subject to a number of disadvantages, particularly that in use the elongated sleeve becomes distorted out of parallelism with the driven shaft so that a binding and non-uniform transmission of torque results. Also, the provision of the hollow driven element leads to excessive distortion in shape of the drive element each time a raised edge of the drive shaft is encountered in its rotation. The system disclosed by that patent is thus one that is incapable of providing smooth and predictable torque transmission and is further one that is not readily adjustable by the addition or removal of a number of drive elements.
A still further approach is shown by U.S. Pat. No. 4,056,953 issued to Furlette, et al on Nov. 8, 1977 for "Torque Limiting Coupling". The combination taught by that patent is one similar to that described in connection with U.S. Pat. No. 3,942,338 of which it is a continuation-in-part. However, the elements for transmitting the torque between drive shaft and hollow, tubular driven member are a plurality of resilient, spherical, members which are loosely disposed along the length of the driving shaft. As is shown by the several drawings, the plurality of spherical elements tend to walk up and down the shaft longitudinally in such way that they come to interact one with the other. Thus, the balls begin to co-act because they are made of a softer material in the shaft. They act in a similar manner to gears and when they apply pressure one against the other, they tend to turn in opposite directions. The net effect of this is to cause rotation of the tubular element in exactly the opposite direction in which it really was intended to turn.
Another patent in which the clutch elements or torque communicating elements are hollow tubes is exemplified by U.S. Pat. No. 3,007,652, issued to Hechtman on Nov. 7, 1961 for "Paper Roll Mounting Mechanism".
The present invention provides an exceedingly simple but effective torque limiting drive system which automatically prevents damage in the event of jam of parts carried by the conveyor. It is possible to vary the drive force being applied by adding one or more spherical drive elements or by varying the hardness of a single drive element being used in the system. Of these two approaches, adding additional elements is the preferable one. A further advantage of the present invention is that in the addition of extra spherical members, they are spaced one from the other and retained by a specialized structure which keeps them in a regular predetermined orbiting type path around the driving shaft.